Couch potatoes have a history that stretches back 7,000 years to when humans first picked up the plough, a study has shown.

It seems that almost as soon as farming was invented, people started to become less active and more puny.

As new innovation and technology contributed to an easier life over the ensuing millennia, humans became further removed from their athletic ancestors.

Scientists tracked the weakening of the human race by studying bones from grave sites across central Europe.

The earliest skeletons examined dated back to around 5,300 BC and the most recent to 850 AD, a time span of 6,150 years.

They show that after the emergence of agriculture, the leg bones of people living along the fertile Danube river valley became progressively less strong.

Men were most affected by the change, which suggested a reduction in mobility and loading.

In other words, they were covering less distance on foot and carrying out lighter physical tasks.

"My results suggest that, following the transition to agriculture in central Europe, males were more affected than females by cultural and technological changes that reduced the need for long-distance travel or heavy physical work," said lead researcher Alison Macintosh, from the department of archaeology and anthropology at Cambridge University.

"This also means that, as people began to specialise in tasks other than just farming and food production, such as metalworking, fewer people were regularly doing tasks that were very strenuous on their legs."

To put the decline in perspective, a comparison was made with previous results from an assessment of the bone strength of Cambridge University undergraduates.

It showed that the earliest male farmers, dating back around 7,300 years, had legs on a par with those of student cross-country runners today.

But within just over 3,000 years, average mobility had dropped to the level of students rated as "sedentary". After this point, the downward spiral towards less activity appeared to slow.

Archaeological evidence shows that intensification of agriculture was accompanied by greater production and complexity of metal goods, technological innovation, and the extension of trade and exchange networks.

At the same time, the bones of both men and women were altered.

Ms Macintosh used a portable 3D laser surface scanner to study the femurs, or thigh bones, and tibiae, or shin bones, of skeletons from Germany, Hungary, Austria, the Czech Republic and Serbia.

She found that male tibiae became less rigid and the bones of both men and women became less strengthened to loads in one direction more than another.

"Both sexes exhibited a decline in anteroposterior, or front-to-back, strengthening of the femur and tibia through time, while the ability of male tibiae to resist bending, twisting, and compression declined as well," said Ms Macintosh, who presented her findings at the annual meeting of the American Association of Physical Anthropologists in Calgary, Canada.

Evidence of declining mobility in women was less consistent than for men. This suggested they were "multi-tasking" or at least undertaking fewer tasks requiring significant lower limb loading.

Iron Age women from between 1,450 and 850 BC bucked the general trend and appeared to have the strongest thigh bones of all the females examined, said Ms Macintosh.

One explanation could be that the Iron Age sample included skeletons from the Hungarian Scythians, known for their Amazonian female warriors who participated in combat.

Ms Macintosh pointed out that if the strength seen in the thigh bones of Iron Age women was due to high mobility, it should be seen in their shin bones as well - but it was not.

"It could be something other than mobility that is driving this Iron Age female bone strength, possibly a difference in body size or genetics," she said.

She concluded: "In central Europe, adaptations in human leg bones spanning this time-frame show that it was initially men who were performing the majority of high-mobility tasks, probably associated with tending crops and livestock.

"But with task specialisation, as more and more people began doing a wider variety of crafts and behaviours, fewer people needed to be highly mobile, and with technological innovation, physically strenuous tasks were likely made easier.

"The overall result is a reduction in mobility of the population as a whole, accompanied by a reduction in the strength of the lower limb bones."

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